Field of the Invention
The present invention relates to an image processing apparatus for correcting colors output from a printer, an image processing method, and a program for generating image processing parameters.
Description of the Related Art
With the improvement in performance of electrophotographic apparatuses in recent years, a certain electrophotographic apparatus has achieved an image quality equivalent to the image quality of printing machines. However, such an electrophotographic apparatus shows a larger color variation amount than printing machines because of the instability peculiar to the electrophotographic apparatus.
To solve this problem, conventional electrophotographic apparatuses are provided with a “single color” calibration technique based on the generation of a lookup table (LUT) for correcting one-dimensional gradation characteristics corresponding to each of cyan, magenta, yellow, and black toners (hereinafter referred to as C, M, Y, and K toners, respectively). A LUT is a table indicating output data corresponding to input data divided at specific intervals. The LUT enables representing nonlinear characteristics which cannot be represented by calculation formulas. “Monochrome” refers to a color represented by using any one of C, M, Y, and K toners. Executing single color calibration corrects the single color reproduction characteristics, such as the maximum density and gradation.
In recent years, Japanese Patent Application Laid-Open No. 2011-254350 discusses a technique for performing “multi color” calibration by using a four-dimensional LUT. “Multi color” refers to colors represented by using a plurality of toners such as red, green, blue, and CMY-based gray. Particularly in electrophotography, even if the gradation characteristics of single colors are corrected by using a one-dimensional LUT, representing “multi color” by using a plurality of toners causes a nonlinear difference in many cases. In this case, executing multi color calibration corrects the color reproduction characteristics of multi color represented by a combination (for example, superposition) of a plurality of color toners.
A method of calibration involving “multi color” will be described below. First of all, to perform “single color” calibration, the method prints out patches on a recording medium such as paper by using single color chart data, and reads the patches by using a scanner or sensor. The method compares data acquired by reading the patches with preset target values, and generates a one-dimensional LUT for correcting differences of the data from the target values. Subsequently, to perform “multi color” calibration, the method prints out patches on a recording medium by using multi color chart data reflecting the generated one-dimensional LUT, and reads the patches by using a scanner or sensor. The method compares data acquired by reading the patches with preset target values, and generates a four-dimensional LUT for correcting differences of the data from the target values.
As described above, high-precision correction was possible by correcting the multi color characteristics through “multi color” calibration, which cannot be corrected only through “single color” calibration.
However, a prior art uses an “L*a*b* color space” for “multi color” calibration and therefore is affected by “distortion” included in the “L*a*b* color space” itself.
Although the “L*a*b* color space” aims to make a correlation with the human visual characteristics, it tends to be difficult to make a correlation in specific hues. For example, a color perceived as “purple” by the human eye may be represented as “blue” in the “L*a*b* color space.”
In “multi color” calibration, the method calculates linear distances of differences in the “L*a*b* color space” and then performs correction. This processing causes the following problem. Although correction is completed on a numerical basis, the human eye perceives that certain colors in a printout have not been sufficiently corrected because of an influence of “distortion” peculiar to the “L*a*b* color space.